What is meant by a closed geothermal exchanger is an exchanger that uses a heat-transfer fluid that circulates not in the surroundings but in sealed underground pipes. The heat-transfer fluid of these systems therefore does not form part of the geological surroundings. As a matter of principle, the heat-transfer fluid that circulates in a closed system is always the same and represents a limited volume. In the case of a closed geothermal exchanger, there is no exchange of matter between the exchanger and the environmental surroundings.
The only geostorage operations performed at the present time are the diffusive geostorage of heat using pressurized water as the heat-transfer fluid (Borehole Thermal Energy Storages—BTES). These geothermal exchangers are made up of coaxial U-shaped tubes, or tubes having another geometry, typically made of high density polyethylene (PEHD) or any other rigid material.
The boreholes in the subsoil in which these tubes are placed are plugged with a hydrated cement the purpose of which is to provide thermal coupling between the wall of the rigid tube that constitutes the exchanger and the wall of the borehole.
Existing closed geothermal exchangers are ill suited to operating temperatures in excess of 100° C. because of the thermal coupling with the encasing rocky massif which is performed by way of a hydrated cement which would experience impediment of its geomechanical properties above and beyond 100° C.
This is why, at the present time, the temperatures of the heat-transfer fluid in operations involving the diffusive geostorage of heat using pressurized water as the heat-transfer fluid in the closed geothermal exchangers are generally comprised between 50° C. and 70° C. The pressure in the exchangers is of the order of ten bar or so.
However, there is a need for closed geothermal exchangers in which the heat-transfer fluid can be at a temperature higher than 100° C.
Thus, for example, for the in-massif storage of electricity by thermal pair as described in document FR 3009613, the “hot” pole of the thermal pair is a heat geostore using closed geothermal exchangers. The heat-transfer fluid in the geothermal exchangers is carbon dioxide (CO2) in a supercritical state, at a temperature that can be as high as 140° C., and at a pressure that can be as high as 120 bar.
The heat-transfer fluid could also be superheated steam. The temperature in the exchanger could then, for example, reach 240° C.